In large extraction plants, such as copper extraction, the adjusting of the solution boundary surfaces and the discharging of the solutions are combined by adopting two collecting chutes provided with solution overflows, which chutes extend vertically over the whole final end of the separation part. Generally there are used two attached chutes, the first of which--in the solution flow direction--is a fixed chute collecting the lighter organic solution as overflow, and the latter is a chute collecting the aqueous solution, provided with an adjustable overflow edge. The heavier solution, i.e. the aqueous solution, is conducted from underneath both chutes through a duct formed in between the chute bottoms and the separation part bottom. From this duct, the aqueous solution turns up and flows in the form of a U-turn in the collecting chute, in a direction that is opposite to the original flowing direction.
The adjustable overflow of the aqueous solution is formed of an outer edge of an aqueous solution chute, known in the prior art, which is constructed of a wall plate extending up to a given height and of another plate moving against it. This structure, provided with a horizontal overflow edge, serves as the basic overflow level, arranged at a height which it is unnecessary to go below from the adjusting point of view. The adjusting range proper locates above this level, and it is taken care of by means of a movable plate part, the overflow edge of which is likewise maintained in horizontal position.
The above described aqueous solution chutes provided with overflow edges include some drawbacks. Two plates moving against each other cannot be made compact, but a remarkable part of the overflow, about 10-40% thereof, passes along some other route than over the overflow edge. Therefore the adjusting of the phase boundary surface works properly only when the solution feeds surpass about half of the amount for which the extraction plants are designed. Moreover, when driving down the process, the above described overflow causes a solution flow to the next process step and thus weakens the extraction results in connection with the next drive up. There is also the danger that the boundary surfaces of the separation parts fluctuate, in which case a phase dispersion carrying impurities and located in between pure phases is transported along with the separated solutions.